1. Field of the Invention
The present invention relates to a positioning apparatus for a circular substrate such as a semiconductor wafer.
2. Description of the Prior Art
Many wafers for manufacturing semiconductor devices have flats formed by linearly cutting off portions of circumferences thereof in order to indicate crystal orientations. When the wafers are mounted on a fabrication machine or a test machine, the flats are usually used to position the wafers (or to eliminate a rotational error and an x-y axes error). One technique to position a wafer by using a flat on the wafer is shown in Japanese Unexamined Patent Publication No. 18713/1983. Recently, as the scale of the wafer is increased, the use of an 8-inch wafer which is larger than a commonly used 6-inch wafer is being considered. In order to mechnically position the 8-inch wafer by using the flat with a high accuracy, a fairly long flat is required. In other words, an area which is cut off to prepare the flat is fairly large. Since a certain number of semiconductor chips could be formed on the cut-off area, it is not advantageous to form a long flat in a large size wafer for the purpose of effectively fabricating a number of chips on the wafer. It has been proposed to fabricate more chips by forming a V-shaped or U-shaped notch in the periphery of the wafer. However, many of the existing wafer positioning apparatus can position a wafer with a flat but cannot position a wafer with a notch.
When a wafer with a notch is to be positioned, the notch must be oriented in a predetermined orientation. The notch is usually much smaller than the cut-off area of the flat and hence it is difficult to precisely detect the notch. Accordingly, even if the wafer is rotated to detect the predetermined orientation of the notch and the wafer is stopped at that position and fixed to the apparatus, the notch position at which the wafer is stopped is not accurate and the wafer is not exactly positioned.